PETROLOGIC INVESTIGATION OF THE NORTHCRAFT FORMATION, SOUTHWEST WASHINGTON: INSIGHTS INTO EARLY CASCADES ARC MAGMATISM

The ~40 Ma Northcraft Formation (NF), located in southwestern Washington, is believed to be one of the earliest expressions of Cascade arc magmatism. The formation consists of >3000 m of subaerial lava flows, which are dominantly porphyritic andesites, and lesser pyroclastic material. Interbeds of breccia, conglomerate, and sandstone are also present (Hagan, 1987). For the present study 21 NF samples and three samples of other early Cascade units (OEC) from SW WA were analyzed chemically and in thin section. The OEC samples are basalts from the Cowlitz and Pe Ell Formations, both undated but regarded as Late Eocene. Goals of the study are to determine: (1) the compositional diversity of the NF, (2) the processes responsible for this diversity, (3) how NF and OEC lavas compare petrologically, and (4) how the NF and OEC lavas compare chemically with younger Cascade lavas.

NF lavas span the basalt, andesite, dacite, rhyolite spectrum (wt. % SiO₂ = 51.1 - 70.5, Mg# = 21-55) and have HFSE depletions and LILE enrichments consistent with arc magmatism, although a Nb/Yb-Th/Yb geotectonic plot suggests only mild arc characteristics. Samples are split between the tholeiite and calc-alkaline series and none exhibit characteristics of primary magmas. Pearce element ratio plots, trace element modeling, and REE plot similarities suggest differentiation by fractional crystallization dominated by plagioclase, clinopyroxene, and olivine, all phases that are visible in thin sections.

The NF has clear compositional differences from both OEC and younger Cascade formation. The OEC samples lack LILE enrichments / HFSE depletions and have generally higher La/Yb than the NF (11.3 – 42.5 vs 6.7 - 12.5). These differences require different mantle sources for the NF and OEC, the latter source not affected by subduction and possibly garnet-bearing. Compared to younger Cascade lavas, NF samples are distinguished by lower Ba/Nb (<25), and a lack of hydrous minerals. These differences are consistent with derivation from a mantle wedge that had experienced minimal modification by slab-derived fluids. Results of this study suggest that over time the composition of the Cascadia sub-arc mantle has evolved in response to the addition of such fluids.